Since a rare earth 123 oxide superconductor (RE-123 super conductor) excels in magnetic-field properties at the temperature of liquid nitrogen as compared with a Bi type superconductor, it can realize a practical high critical current density (Jc) in a high magnetic field. Accordingly, if the practical use of a wire material thereof is achieved, in addition to excellent properties at high temperature regions, there is an economically extraordinary advantage because a production method without using silver, which is noble metal, is possible, and, since liquid nitrogen can be used as a refrigeration medium, cooling efficiency is enhanced to several tens to several hundreds times. The result makes it possible to apply a superconductive wire material to instruments, to which the application has been conventionally impossible from the economic perspective. Thus, significant expansion of the application and market of superconductive instruments is foreseen.
RE-123 super conductors (particularly, Y-123 super conductor, Y:Ba:Cu=1:2:3 at mole ratio) has an orthorhombic crystal system. Therefore, in order to make them exert characteristics of the material in conductive characteristics, it is required not only to align CuO planes of the crystal, but also to align in-plane crystal orientations. The reason is that a slight misalignment of orientations creates a twin boundary to lower conductive characteristics.
A manufacturing method of a wire material of the Y-123 super conductor while enhancing in-plane alignment and aligning the in-plane orientation of crystals thereof has the same way as a manufacturing method of a thin film. That is, it is possible to enhance the in-plane alignment degree and orientation of crystals of Y-123 super conductor by forming an intermediate layer having been enhanced in the in-plane alignment degree and the orientation on a tape-formed metal substrate, and using the crystal lattice of the intermediate layer as a template.
Further, it has been proved that Jc of a superconductor depends on crystallinity and surface smoothness of an intermediate layer, and that characteristics thereof changes sensitively and significantly according to conditions of an underlying layer.
With regard to production techniques of above-described biaxially aligned metal substrate in which an in-plain aligned intermediate layer is formed on a tape-formed metal substrate, there are known such methods as an SOE (Surface-Oxidation Epitaxy) method, an ISD (Inclined Substrate Deposition) method, an IBAD (Ion Beam Assisted Deposition) method and a RABiTS (Rolling Assisted Biaxally Textured Substrate) method, and there are many reports about Y-123 superconductive wire materials having a Jc of more than 106 Å/cm2 by forming an intermediate layer, whose in-plane alignment degree and orientation have been enhanced, on a non-aligned or aligned metal tape.
Among these, in formation of the intermediate layer in IBAD and RABiTS methods, a vacuum process based on a vapor phase method such as a PLD (Pulse Laser Deposition) method is used, and in the IBAD method, a combination of hastelloy/YSZ/Y2O3, and, on the other hand, in the RABiTS method, a combination of Ni/CeO2/YSZ/CeO2 or the like are generally employed as a biaxially aligned metal substrate, which have such advantage that they can give a dense and smooth intermediate layer film (for example, refer to Non-patent Document 1).
There are many examinations about materials for the intermediate layer. Among these, a CeO2 intermediate layer is known as one of the best intermediate layers since it has a good consistency of crystal lattice with a YBCO layer (Y—Ba—Cu—O superconductive layer) and a low reactivity with a YBCO layer, and many results are reported.
As described above, CeO2 has excellent properties as an intermediate layer when forming a YBCO layer on a metal substrate. However, since a CeO2 film is apt to easily crack because of differences in thermal expansion and the like from a metal substrate and can not be formed in a thick film, an intermediate layer of YSZ (yttrium-stabilized zirconia) or the like must be interposed between a CeO2 film and a metal substrate to form a YBCO layer thereon, as is the case for the RABiTS method.
In order to solve the problem, the present inventors formed an intermediate layer composed of a cerium-based oxide incorporating cerium with 1 type or 2 or more types of rare earth elements, for example, of a solid solution generated between CeO2 and Gd2O3 on a metal substrate by an MOD (Metal Organic Deposition Processes) method, and formed a rare earth oxide superconductive layer (RE superconductive layer) on the intermediate layer, thereby succeeded in forming a RE superconductive layer excelling in superconductive properties on an intermediate layer that can prevent cracking, excels in crystallinity and surface smoothness, and is capable of low temperature synthesis (refer to Japanese Patent Application Nos. 2003-129368 and 2003-129369).
However, it was revealed that a YBCO layer formed on the intermediate layer by an MOD method shows a critical temperature (Tc) lower than the Tc of 90 K fundamentally belonging to a YBCO layer by around 10 K.
This is attributable to diffusion of a Ni element in the substrate into the superconductive layer, thereby substituting for a Cu element constituting the RE-123 super conductor.
Therefore, it was revealed that, although an intermediate layer excelling in lattice consistency, that is, crystallinity and surface smoothness and being capable of preventing cracking can be obtained when forming an intermediate layer of cerium-based oxide incorporating cerium with 1 type or 2 or more types of rare earth elements, for example, Ce—Gd—O by an MOD method as an intermediate layer on a metal substrate, there is a room for improvement as regarding an inhibiting effect on diffusion of an element constituting a substrate.    [Non-patent Document 1] A. Goyal et al., Physica C, 357-360 (2001) 903.